In radio communications systems, data is transmitted via electromagnetic waves. The electromagnetic waves are transmitted via antennas, with the carrier frequencies being in the frequency band (or bands) intended for the respective system. In addition to the requirement to restrict the dimensions of the antenna to fit into the small sizes of the mobile radio transmitting and receiving devices, there is also an increasing requirement for the capability to transmit and receive in multiple different frequency bands, thus, giving the mobile radio devices access to greater bandwidth.
Tunable antennas, therefore, are desirable given the current demand for bandwidth in today's mobile radio designs. A planar inverted F-type antenna (PIFA) is one example of a tunable antenna. A typical PIFA antenna may be tuned to have three resonances that correspond to Global System for Mobile Communications (GSM)/Wide Band Code Division Multiple Access (WCDMA) bands. For example, a typical PIFA antenna may have a first resonance with a bandwidth from 824 MHz to 960 MHz at −6 dB (low band) and two other resonances with a bandwidth from 1710 MHz to 2170 MHz at −6 dB (mid and high bands). Each resonance in a PIFA antenna is set by the effective length of the current flow on the antenna pattern surface and can be expressed by:
                              L          +          W                =                              λ            air                                4            ⁢                                          ɛ                reff                                                                        Eqn        .                                  ⁢                  (          1          )                    
where λ is the wavelength in air;                L+W is the two dimensional effective current flow on the antenna pattern; and        ∈reff is the effective dielectric constant, which can further be expressed by:        
                              ɛ          reff                =                                                            ɛ                r                            +              1                        2                    +                                                                                          ɛ                    r                                    -                  1                                2                            ⁡                              [                                  1                  +                                      12                    ⁢                                          H                      W                                                                      ]                                                    -                              1                2                                                                        Eqn        .                                  ⁢                  (          2          )                    
where ∈r is the dielectric constant of the antenna's substrate.
Tuning down (towards lower frequencies) the third high band resonance of a PIFA antenna can be done, but typically only by adding matching components that reduce the total efficiency of the antenna.